Microevolution explains what cancer is, how it develops and how to eradicate it. Cancer occurs in multi-celled organisms when cells escape the body's controls and behave like their single-celled ancestors. Such changes, triggered by oxidative damage, result in faulty cell division. Animals and plants have developed ways to stop their cells reverting to primitive forms. Hence, anticancer substances are common throughout nature. Therapies based on these take advantage of metabolic differences between cancer cells and healthy cells, to destroy cancer while helping healthy cells. Clinical trials are needed to test such non-toxic therapies. Biological research suggests that cancer is a treatable condition. Although current data is not sufficient to indicate the degree of life extension achievable, many terminal patients might die of other causes, before the cancer kills them. Cancer patients deserve to be offered this opportunity.